In OFDM or OFDMA systems, such as IEEE 802.16d/e system, the preamble is designed in consideration to support systems working in various frequency multiplexing schemes including the scheme in which the multiplexing factor is 1. Cell ID (or IDCell) and segment number embedded in the preamble can be used as identification of the cell and the sector respectively, which are also utilized as an item of permutation generator for subcarrier mapping and as an initialization vector of pseudo-random binary sequence (PRBS) generator for subcarrier randomization. Only after the preamble pattern has been correctly detected, the MS can then correctly de-map and de-randomize the subcarrier, and further demodulate/decode OFDMA symbols following the preamble in a downlink frame.
There are altogether 114 preamble codes or preamble patterns specified in the IEEE 802.16d/e system. These preamble sequences are generated through computer searching. The preamble codes have therebetween low cross-correlation values in frequency domain and low peak to average power ratio (PAPR) in time domain. For scalable OFDMA systems such as IEEE 802.16e, the length of a preamble code is different for various FFT sizes.
The preamble codes are modulated to specific pilot subcarriers according to the corresponding segment number by:PreambleCarrierSetseg=seg+3k  (1)where PreambleCarrierSetseg specifies all subcarriers allocated to the preamble code, seg is the segment number indexed from 0 to 2, and k is a running index from 1 to the length of the preamble code.
Each preamble code has its unique preamble index, IDCell and segment number. For instance, for FFT-2048 (indicating that the FFT size equals to 2048; FFT-512 and FFT-1024 appearing hereinafter indicate that the FFT size equals to 512 and 1024, respectively), the preamble code with Index=0, IDCell=0 and Segment=0 is:
Wk=0xC12B7F736CFFB14B6ABF4EB50A60B7A3B4163EA3360F697C45075 997ACE17BB1512C7C0CEBB34B389D8784553C0FC60BDE4F166CF7B048 56442D97539FB915D80820CED D858483 (in Hex type).
The preamble code is modulated by 2√{square root over (2)} boosted BPSK modulation as:
                    {                                                                              Re                  ⁢                                      {                                          X                      ⁡                                              (                        k                        )                                                              }                                                  =                                                      2                    ⁢                                                                  2                                            ·                                                                        X                          ′                                                ⁡                                                  (                          k                          )                                                                                                      =                                      2                    ⁢                                                                  2                                            ·                      2                                        ⁢                                          (                                                                        1                          /                          2                                                -                                                  W                          k                                                                    )                                                                                                                                                                Im                  ⁢                                      {                                          X                      ⁡                                              (                        k                        )                                                              }                                                  =                0                                                                        (        2        )            where X′(k) is the preamble code after the BPSK modulation, X(k) is the boosted preamble code, and Re(•) and Im(•) are respectively real part and virtual part acquisition calculations.
After modulation, these modulated symbols are mapped to pilot subcarriers and zero is used to fill the unallocated subcarriers. As should be noted, however, if a DC subcarrier is precisely included in a pilot position to which the preamble code corresponds, no modulation will be made and the code corresponding to this position will be discarded. FIG. 1 shows the structures of the preamble of FFT-512 and FFT-1024 based on the IEEE 802.16e, wherein a preamble code is modulated at intervals of every three subcarriers except the left and right virtual carriers. A base station (BS) or a sector of a BS has its unique preamble code and is transmitted in the head of every frame.
FIG. 2 shows cross-correlation characteristics among preamble codes. As shown in FIG. 2, the cross-correlation values among different preamble codes are low, but the self-correlation values of each preamble code are high. Therefore, in the receiver of a mobile station, preamble index can be detected by the conventional cross correlation between a post-FFT preamble and the known preamble codes like the detection of the conventional pseudo noise (PN) code. Detection is also possible by the differential cross correlation method in view of preventing channel fading and timing offset. After the correlation calculation, the code with the maximum correlation value is taken as the preamble index of the target BS.
However, during the initial synchronization period, there is usually a carrier frequency offset (CFO) between the BS and the MS. The oscillator of the MS is generally not provided with high accuracy out of commercial considerations. As a result, the carrier frequency offset is larger than one subcarrier spacing, that is to say, there is an integer carrier frequency offset (ICFO) between the MS and the BS. In this case, preamble sequence in the frequency domain will be shifted along the subcarrier axis. Conventional preamble detection method cannot work in this situation. Consequently, there is a need for a preamble detection algorithm and a preamble detection apparatus operating accurately in large carrier frequency offset environment (especially in integer carrier frequency offset environment), and a need for an integer carrier frequency offset (ICFO) estimator.
Additionally, the MS needs to choose a BS with the best channel condition as the target BS for initial network access. Conventional BS or cell selection method only makes decision from the preamble detection result of one frame. However, when an MS approaches a cell edge, it can receive signals from a plurality of BSs. The fading paths between the MS and each BS are normally independent. The different fading paths will easily mislead selection of the target BS if only one preamble detection is relied upon. Accordingly, there is also a need for a reliable cell selector during the initial network access.